The present invention relates to a shoulder prosthesis, of the type comprising a humeral piece intended to be fixed in a humerus to replace at least the head thereof and comprising a hollow tubular element which must be fixed in the humerus substantially along the longitudinal axis thereof and a cylindrical rod, the upper end of which carries a ball cooperating with a glenoid bearing surface to form the shoulder joint and the lower end of which is disposed in said tubular element, this and the rod being able to slide relative to each other, the glenoid bearing surface consisting of a glenoid piece which comprises an anchoring element intended to be inserted into a cavity formed in the scapula and a female hemispherical element, this latter cooperating with the male ball of the humeral piece.
The humeral piece must be fixed in the upper end of the humerus to replace the head and if necessary the greater tuberosity thereof; the glenoid piece, occasionally referred to as the scapular piece, must be introduced into the scapula to replace the glenoid cavity thereof.
As is the case for any joint prosthesis, the fixation of each piece of this prosthesis in the bones poses a major problem. In fact, such fixation will always be of limited strength, because there cannot be total microscopic union between the living bone tissue and the inert material of the prosthesis. At present, there are two possibilities of improving such fixation: either to use prosthesis of rough surface such that the bone generates bony tissue which molds to the different uneven areas; or to use a plastic substance, which penetrates into the anfractuosities of the bone and thus renders the piece integral with the bone, such as, for example, methyl methacrylate.
Despite the use of one or other of these possibilities, the bone/prosthesis junction still remains a zone of weakness. However, it is indispensable that this junction be as strong as possible in order to permit the movements of the joint and to prevent the pains which are caused in the case of poor fixation. Moreover, working loose between the prosthesis and the bone causes destruction of the bone and consequently leads to detachment of the prosthesis.
In the case of a shoulder prosthesis, consisting of a humeral piece and a glenoid piece, the glenoid piece is that which is most susceptible to being separated from the scapula; in fact, this bone is of very slight thickness, approximately 1 cm, and of small surface area, approximately 4 to 6 cm.sup.2. Moreover, the glenoid piece is subjected to upward thrusts caused by the muscles and the supports of the hand, and to downward tensions caused by the weight of the arm itself as well as by objects held in the hand. In the normal state, there exists a muscle group which is located between the acromion, a process of the scapula, and the head of the humerus, and which acts as a buffer and opposes the movement generating an upward pressure. Unfortunately, this muscle group wears out, and it is precisely when this group has worn out that it becomes necessary to contemplate the installation of a prosthesis, which therefore cannot use this group to prevent the upward movements.
One of the objects of the present invention is to provide a shoulder prosthesis comprising a humeral piece and if necessary a glenoid piece, of the type defined in the foregoing, the fixation of which in the scapula is of longer duration.
Another object of the present invention is to provide such a prosthesis which is stable, i.e., such that the pieces thereof are not susceptible to becoming displaced relative to each other, as in the case of a dislocation.